Many internal combustion engine systems incorporate exhaust gas recirculation (“EGR”) systems. Functionally, the EGR system redirects a portion of exhaust gas back to the cylinders of the internal combustion engine for combustion. However, exhaust gas is inert and therefore does not partake in the combustion process in the engine. Accordingly, the recycled exhaust gas has many other effects on the internal combustion engine.
Primarily, in a gasoline spark-ignition engine, the addition of exhaust gas tends to lower combustion temperatures. Lower combustion temperatures decrease the production of nitrogen (“NOx”) and carbon (“COx”) oxides. Accordingly, emission characteristics for a gasoline internal combustion engine tend to improve with EGR systems. Moreover, because of the lower combustion temperatures, the likelihood of the internal combustion engine “knocking” decreases (i.e., premature combustion within the cylinder). However, there are limits to the amount of EGR that can be added to the engine.
Because EGR displaces oxygen otherwise used for combustion, there can be an increase in particulate matter produced (e.g., non-combusted fuel particles). Similarly, there typically is a reduction in output power from the engine in EGR systems because of the reduction in combustible matter. Moreover, due to the decrease in oxygen, ignition can be very difficult. As such, one strategy has been to retard ignition timing to provide more time for combustion to occur prior to the exhaust stroke.
Another strategy has been the utilization of a dual coil ignition system. In a dual coil ignition system, the ignitor (e.g., a spark plug) is coupled to two sets of primary and secondary windings that are able to provide the spark plug with an extended duration of spark current. Accordingly, a longer spark (i.e., ignition event) can be used to combust difficult combustion mixtures (e.g., having too much EGR in the cylinder).